x = [4, 6, 2, 1, 7, 9]

# 错误示范
y = x.sort()
print(y)  
#output: None

y = x.copy()
y.sort()
print(y)
#output:[1, 2, 4, 6, 7, 9]

z = sorted(x)
print(z)
#output:[1, 2, 4, 6, 7, 9]

import os
fls = os.listdir(".")
fls.sort()
print(fls) 
#output:['analysis.md', 'code.py', 'output.txt']
